1. Field of the Invention
The present invention concerns a synchronization signal generating circuit for an optical scanning device that is used in a laser beam printer, etc.
2. Description of the Related Art
An optical scanning device that is used in a laser beam printer, etc. is arranged so that a laser beam from a light source unit with a laser diode is deflected and scanned in a predetermined direction and in a predetermined angular range by a rotating polygon mirror (polygon mirror) and this deflected and scanned laser beam is converged onto the photosensitive surface of a photosensitive drum via an f.theta. lens to thereby obtain a scanning light beam that moves at uniform speed in the direction of the axis of rotation of the photosensitive drum.
In a laser beam printer, the laser beam is optically modulated at the light source unit of the optical scanning device on the basis of drawing data, the photosensitive drum is rotated based on the timing of scanning operation by the optical scanning device, an electrostatic latent image that corresponds to the drawing data is formed on the outer peripheral surface (light receiving surface) of the photosensitive drum, toner is adsorbed onto the outer peripheral surface of the photosensitive drum in correspondence with the electrostatic latent image, and this toner image is transferred (drawn) onto the recording paper.
In such an optical scanning device for a laser beam printer, a synchronization signal (horizontal synchronization signal) HSYNC is generated for recognition of the scanning starting position on the light receiving surface of the photosensitive drum, in other words, the drawing starting position on the recording paper, and arrangements are made to detect synchronization signal HSYNC and to start drawing after the elapse of a certain amount of time from synchronization signal HSYNC.
As shown in FIG. 12, a photodiode 6 is installed at a position outside the drawing range and separated from the drawing starting position by a predetermined distance in the main scanning direction, and synchronization signal HSYNC is generated and detected on the basis of a signal output upon photoelectric conversion of the laser beam received by the light receiving surface of photodiode 6.
The conventional synchronization signal generating circuit 100, which detects synchronization signal HSYNC in this case, is comprised of photodiode 6, a resistor 41, which is the load circuit of photodiode 6, an amplifier 35, a comparator 38, and a reference voltage generator 39.
As shown in FIG. 13, when the laser beam is illuminated onto the light receiving surface of photodiode 6, a current, in other words, a signal of a magnitude that corresponds to the amount of light received is output from photodiode 6. This signal is converted into voltage by the load circuit (resistor 41), amplified by amplifier 35, and then input into comparator 38. At comparator 38, the abovementioned signal is compared with a reference voltage (threshold voltage) that is output from reference voltage generator 39 and a rectangular synchronization signal HSYNC, which rises at crossing point 42 and falls at crossing point 43, is generated and output. The timing at which this synchronization signal HSYNC falls is used for the detection of the scanning starting position on the light receiving surface of the photosensitive drum, in other words, the drawing starting position on the recording paper.
However, the signal output from photodiode 6 may fluctuate because of fluctuations of the amount of light received by photodiode 6 due to fluctuations of the laser beam output, tilting of the faces of the rotating polygon mirror, etc., mixing in of unwanted signal components (noise), influences of the fluctuations, etc. of the DC component of the circuit due to the dark current, etc., and this may cause the pulse width of synchronization signal HSYNC, in other words, the timing at which synchronization signal HSYNC rises and falls to fluctuate and thus make difficult highly precise detection of synchronization signal HSYNC.
The above can cause, for example, variations in the timing of the rise or fall of synchronization signal HSYNC, disturbance of the drawing starting position, and the lowering of the quality of drawing (printing).